This invention relates to a method for correcting deflection distortion which develops in an apparatus for delineating patterns on a sample by scaning over the sample with a charged particle beam, such as electron beam or an ion beam; for example, an apparatus for electron beam lithography.
In general, in an apparatus for electron beam lithography and the like, the deflection field is distorted on account of fabrication errors in the apparatus itself, the installation errors of an electron lens and a deflector, and so on. That is, when the deflection data is in the form of a checkered lattice, the phenomenon that a delineated result becomes a distorted lattice takes place. In order to permit the registration between masks prepared by different apparatuses, such deflection distortion must be eliminated.
Especially in an apparatus for mask exposure, a correction method as illustrated in FIG. 1 has been proposed by G. A. C. Jones and G. Owen: Journal of Vacuum Science and Technology, 15(3), May/June, 1978, for correcting the distortion. As seen in FIG. 1, a single reference mark 1 is provided on a part of a work table 2. The work table 2 is moved to change the relative position of the mark with respect to a beam axis, and this position is measured with laser interferometers 3 and 4. Simultaneously therewith, while applying deflection data from a control computer (CPU) 7 to a deflector 6, the area of the mark 1 is scanned by an electron beam 5 so as to detect the position of the mark 1. Thus, the distortion of the deflection is measured. If the data can be corrected so that the distortion apparently disappears, it is determined that the correction of the deflection distortion has been accomplished.
Although this distortion correction method provides a satisfactory precision for the exposure of the mask or a wafer of small diameter, the correction method alone is unsatisfactory in, for example, an apparatus for the direct exposure of a device which has dimensions of submicrons requiring a high precision within .+-.0.1 .mu.m . This is because the wafer itself generally has a deformation. By way of example, when the wafer is greater than 3", the magnitude of the deformation thereof becomes greater than 200 .mu.m in the standard semiconductor manufacturing process. Accordingly, the focus, deflection gain, deflection rotation, barrel or pincushion, and other distortions need to be corrected for every field to be delineated in the "step and repeat" process.
As an expedient for reforming the deformation of the wafer, there has been proposed an electrostatic chuck system as suggested by G. A. Wardly: Review of Scientific Instrument, Vol. 44, 1506, 1973. However, this system has not been put into practical use in view of the structural complexity etc., thereof.